I. Field of the Invention
The present invention relates to a cloth prepreg and process for producing the same. More particularly, the present invention relates to a cloth prepreg which gives a fiber-reinforced plastic (hereinafter also referred to as "FRP") having high toughness and high modulus of elasticity as well as high thermal resistance, low water absorption and good solvent resistance, and which hardly forms pores in the FRP plate prepared by the so called honeycomb co-curing using the prepreg, and to a prepreg which gives an FRP having a good burn-through property.
II. Description of the Related Art
Epoxy resins are widely used in various industrial fields as molded articles, laminated articles, adhesives, sealants and the like exploiting their excellent mechanical properties, high chemical resistances and the like. Fiber-reinforced composite materials which have reinforcing fibers and matrix resins as indispensable constituents are used as structural materials of airplanes, automobiles and the like, and epoxy resins are widely used therein. Particularly, carbon fiber-reinforced plastics (CFRP) are conventionally used as structural materials of airplanes. To reduce the weight of the airplanes, honeycomb sandwich panel structure is often employed. The honeycomb sandwich panel structure is prepared by laying up prepregs on both sides of a honeycomb core made of aramide paper or the like (in some cases, via adhesive films), and the curing of the prepregs and the adhesion of prepregs to the honeycomb core are simultaneously carried out (this curing is called honeycomb co-curing). Thus, good co-curing properties are demanded for the prepregs.
It is also demanded that pores be not formed in the FRPs of the honeycomb sandwich panel. Since no pressure is applied to the regions of the prepregs on and under the inner space of the hexagonal cells in the honeycomb core, pores are likely to be formed in the interlayer zones and within one prepreg layer. If a molded plate having such pores are used to constitute a spoiler of an airplane, water is accumulated in the pores and when the airplanes flies at high altitude, the water is frozen to crack the skin (i.e., FRP of the sandwich panel). By repeating this, water comes to enter the honeycomb core and the physical properties of the panel is degraded, so that safe operation of the airplane is jeopardized. In other words, if the porosity of the FRPs are high, the reliability as a fiber-reinforced composite material is degraded.
There is the following prior art concerning the prepreg employing carbon fibers as reinforcing fibers and concerning matrix resins, which are designed to be used for the honeycomb co-curing.
Japanese Patent Publication (Kokoku) Nos. 63-30925 and 1-29814 disclose the following epoxy resin composition by which good direct adhesion between the prepregs and the honeycomb core and good composite properties of the cured plate as a surface material, especially, high interlayer shearing strength (ILSS) are attained. That is, as the epoxy resin, three types of epoxy resins, that is, bisphenol A type, novolak type and glycidylamine type are selected. The resin composition further comprises a reaction product between liquid butadiene-acrylonitrile copolymer having carboxyl groups at both ends and a glycidylamine type epoxy resin, as well as nitrile rubber and dicyandiamide as a curing agent. The content of each component is made within a prescribed range by which the above-mentioned properties are stated to be attained. Japanese Patent Publication (Kokoku) No. 62-28167 discloses as a matrix resin suited for hybrid reinforcing fibers including carbon fibers and aromatic polyamide fibers, a composition comprising an epoxy resin, a reaction product between liquid butadiene-acrylonitrile copolymer having carboxyl groups at both ends and an epoxy resin, and dicyandiamide, as well as liquid or semi-solid polyamide having amino group at its end. Japanese Laid-open Patent Application (Kokai) Nos. 58-82755, 58-83022 and 58-83031 disclose that by employing a composition containing a reaction product between liquid butadiene-acrylonitrile copolymer having carboxyl groups at both ends and an epoxy resin, and both dicyandiamide and diaminodiphenylsulfone as curing agents, direct adhesion between the prepregs and the honeycomb core is promoted, especially at a high temperature.
Japanese Laid-open Patent Application (Kokai) Nos. 57-21427 and 57-21450 disclose that a prepreg prepared by impregnating into fibers a resin solution obtained by dissolving a composition containing an epoxy resin, a reaction product between liquid butadiene-acrylonitrile copolymer having carboxyl groups at both ends and an epoxy resin, nitrile rubber and a curing agent represented by dicyandiamide, in acetone-methyl Cellosolve mixed solvent or the like gives excellent adhesion strength, high impact strength and high bending strength to the honeycomb sandwich panel.
Japanese Laid-open Patent Application (Kokai) No. 57-49646 discloses that a prepreg employing a resin composition comprising an epoxy resin, nitrile rubber and a high molecular epoxy resin having a molecular weight of not less than 10,000 exhibits good direct adhesion to the honeycomb core, so that it gives high adhesion strength and high bending strength to the honeycomb sandwich panel. In the examples thereof, dicyandiamide is used as a curing agent and prepregs are prepared by dissolving the composition in acetone-methyl Cellosolve mixed solvent or the like.
Japanese Laid-open Patent Application (Kokai) No. 58-84825 discloses a prepreg in which an epoxy resin composition containing a bisphenol A type epoxy resin, an urethane-modified epoxy resin, an epoxy resin having N,N-diglycidylamino group, a brominated bisphenol A type epoxy resin and dicyandiamide is impregnated. It is described that the honeycomb sandwich panel employing this prepreg has excellent high heel resistance and high bending strength.
However, the above-mentioned prior art does not aim at reducing the porosity in the interlayer zones.
Further, if diaminodiphenyl sulfone alone is used as a curing agent which gives high thermal resistance, pores are much more easily formed in the interlayer zones than in cases where dicyandiamide is used as a curing agent, and this problem is very difficult to solve.